Graphite has long been recognized as a material which exhibits superior quality for sealing and gasket applications. These characteristics include high thermal stability, low thermal conductivity, natural lubricity, resistance to chemical degradation, conformability, and resilience.
Graphite has typically been provided in the form of calendared sheets made with expanded intercalated flake graphite worms. Intercalated flake graphite is made by treating natural or synthetic graphite flakes with an intercalating agent such as fuming nitric acid, fuming sulfuric acid, or mixtures of concentrated nitric acid and sulfuric acid. The intercalated flake graphite is then expanded at high temperatures to form a low-density, worm-like form of particulate graphite having typically an 80–100 fold increase in size over the flake raw material. U.S. Pat. No. 3,404,061 describes the production of intercalated flake graphite as an intermediate step in the production of expanded intercalated graphite. Expanded intercalated graphite worms have thin structural wall and are light-weight, puffy, airy, and elongated bodies.
These characteristics lead to exceedingly difficult volumetric, handling, and use problems. Because of these characteristics, expanded intercalated graphite worms typically are calendared to produce sheets of graphite. Calendared graphite is commercially available as GRAFOIL brand sheets. The sheets have a uniform density and a uniform thickness. The sheets are generally available in several standard thickness and densities. The sheet is die-cut to form a gasket. To provide increased tensile strength, a layer of mylar adhesive is applied to one surface of the sheet. The mylar allows the gasket to be applied to an annular metal disk. Gaskets manufactured with calendared graphite sheet typically are used for sealing purposes in high pressure, high temperature fluid flow applications. While such gaskets perform sealing functions, there are drawbacks to their use. Cut calendared graphite sheet particularly provides open edges which is susceptible to high pressure attack from the fluids being sealed by the gasket.
Further, the expanded intercalate graphite worms are extremely light and puffy. A significantly large volume of the worms is required to produce a relatively thin layer of gasket material. There is an approximate 100 to 1 ratio between the volume of expanded worms and compressed worms. The worms being extremely lightweight, are difficult to handle. The slightest air current quickly stirs up the worms. Accordingly, expanded intercalated graphite typically was calendared to form graphite sheets.
U.S. Pat. No. 5,785,322 describes the use of the expanded intercalated graphite worms in forming a seamless gasket for high pressure, high temperature fluid flow applications. Gaskets of this type have superior performance without the drawbacks of conventional sheet-formed gaskets. The manufacture of these improved gaskets however is difficult, expensive, and labor intensive. The manufacturing problems arise from the characteristics of expanded intercalated graphite worms discussed above. The manufacturing process involves manually loading a die with expanded intercalated graphite worms, which are then compressed with a hydraulic press. A significant amount of worms must be loaded in the die, because of the high expansion volume of the worms. A typical ⅛ inch thick gasket requires between 10 and 12 inches of expanded intercalated graphite worms. Yet the mass of the worms is small, and typical gaskets have about a gram of worms on the opposing sides.
While the resulting gasket exhibits superior sealing performance, air may be entrained in the gasket or some portions may have differing densities due to the movement or uneven provision of worms to the die.
Accordingly, there remains a need in the art for an apparatus and method for manufacturing seamless gaskets with lightweight expanded materials. It is to such that the present invention is directed.